1. Field of the Invention
The present invention relates to a wide-band tuner of a television receiver, and more particularly to a wide-band tuner having a temperature-compensated microstrip resonator arrangement.
2. Description of the Prior Art
FIG. 1 shows a block diagram of a tuner of a television receiver which converts a received frequency to an intermediate frequency by a well-known double-superheterodyne system.
The tuner comprises an RF signal input terminal 1, a band-pass filter 2, a variable attenuator 3 for controlling a gain by an AGC signal, an RF wide-band amplifier 4, a first mixer 5, a first local oscillator 6, a narrow-band band-pass filter 7, a first intermediate frequency amplifier 8, a second mixer 9, a second local oscillator 10, a second intermediate frequency amplifier 11 and an intermediate signal output terminal 12.
The received RF signal f.sub.RF of 50-900 MHz applied to the RF signal input terminal 1 is selected by the band-pass filter 2 which also suppresses a harmonic signal interference and an intermodulation interference, and the selected RF signal is attenuated by the variable attenuator 3 by the AGC signal which is proportional to the magnitude of the RF signal, and it is then amplified by the RF wide-band amplifier 4 and then it is supplied to the first mixer 5. The first mixer 5 converts a desired RF signal of the input RF signal to a first intermediate frequency signal f.sub.IF1 by an oscillation signal f.sub.OSC1 of the first local oscillator 6. For example, when f.sub.RF =500 MHz, the signal f.sub.OSC1 of 2500 MHz is oscillated so that the signal f.sub.IF1 of 3000 MHz is produced. The signal of 3000 MHz of the signal converted by the first mixer 5 is selected by the narrow-band band-pass filter 7 and a signal which disturbs an image and harmonic components of the signal f.sub.OSC1 are attenuated by the filter 7, and the output of the filter 7 is amplified by the first intermediate frequency amplifier 8, and the output of the amplifier 8 is supplied to the second mixer 9. The second mixer 9 converts the first intermediate frequency signal from an oscillation signal f.sub.OSC2 of the second local oscillator 10 to a second intermediate frequency signal, that is, a normal intermediate frequency signal f.sub.IF. When the intermediate frequency signal f.sub.IF is 57 MHz, the oscillation signal f.sub.OSC2 is 3057 MHz. The intermediate frequency signal f.sub.IF thus derived is selectively amplified by the intermediate frequency amplifier 11 and an output thereof is taken out from the intermediate frequency signal output terminal 12.
A UHF/VHF band tuner which has an intermediate frequency in a 3 GHz band and uses a YIG filter is disclosed in the U.S. Pat. No. 3,939,429 issued on Feb. 17, 1976. The YIG filter is difficult to manufacture, expensive and locks practicability.
A variation of frequency of the circuit due to a temperature change is now discussed. The frequency f.sub.IF of the intermediate frequency signal is expressed by a formula: (1): EQU f.sub.IF =f.sub.OSC2 -(f.sub.RF +f.sub.OSC1) (1)
From the formula (1), a variation .DELTA.f.sub.IF of the frequency f.sub.IF when a surrounding temperature T changes by .DELTA.T is expressed by a formula (2); EQU .DELTA.f.sub.IF =.DELTA.f.sub.OSC2 -.DELTA.f.sub.OSC1 ( 2)
where .DELTA.f.sub.OSC2 and .DELTA.f.sub.OSC1 are variations of f.sub.OSC2 and f.sub.OSC1, respectively. From the formula (2), if the variations of f.sub.OSC2 and f.sub.OSC1 are equal, the variation of the intermediate frequency f.sub.IF can be made sufficiently small even if the variations of f.sub.OSC2 and f.sub.OSC1 are large. Since .DELTA.f.sub.OSC2 and .DELTA.f.sub.OSC1 depend on the magnitudes of f.sub.OSC2 and f.sub.OSC1, respectively, the magnitudes of f.sub.OSC2 and f.sub.OSC1 must be of substantially the same order.
The output level of the intermediate frequency signal is determined by a frequency relation of the narrow-band band-pass filter 7 and the second local oscillator 10. In order to extract the intermediate frequency signal at an optimum condition, it is essential that the absolute values of the frequency variations of the band-pass filter 7 and the second local oscillator 10 are equal. The formula (1) can be expressed by a center frequency f.sub.IF1 of the pass band of the narrow-band band-pass filter 7 as shown by a formula (3) and a variation of the formula (3) to the temperature change is expressed by a formula (4). Thus, if a frequency variation .DELTA.f.sub.IF1 of the pass band of the narrow-band band-pass filter 7 and the frequency variation .DELTA.f.sub.OSC2 of the second local oscillation frequency are equal and f.sub.IF1 and f.sub.OSC2 are of substantially the same order, the variation of the output level of the intermediate frequency signal can be made sufficiently small. EQU f.sub.IF =f.sub.OSC2 -f.sub.IF1 ( 3) EQU .DELTA.f.sub.IF =.DELTA.f.sub.OSC2 -.DELTA.f.sub.IF1 ( 4)
From the above, if the frequency variations of the first local oscillator 6, the narrow-band band-pass filter 7 and the second local oscillator 10 to the temperature change are equal, the variation of the intermediate frequency and the variation of the output level of the intermediate frequency signal can be made sufficiently small even if the above frequency variations are large.
The first intermediate frequency f.sub.IF1 is usually set to be two times, and preferably four times as high as the highest frequency f..sub.RF max of a reception RF signal which can be received by the tuner in order to avoid the interference which would otherwise occur in the first mixer 5. Accordingly, the frequency f.sub.IF1 is set to a frequency in the 3 GHz band, as described above. In this case, f.sub.OSC1 is 2100-2950 MHz, f.sub.OSC2 is 3057 MHz and a resonator is preferably constructed by a microwave integrated circuit (MIC) which uses a microstrip line. The circuit can be miniaturized by utilizing a high dielectric constant substrate such as an alumina-ceramic substrate. Accordingly, the circuit can be constructed by the microstrip line without using capacitors and coils which were required in the prior art tuner, and hence mass-productivity is improved.
As a resonator which uses the microstrip line, for the first local oscillator, the narrow-band band-pass filter or the second local oscillator, a grounded end shortened resonator shown in FIG. 2 is disclosed in a Japanese article by Ogawa et al entitled "All-Band Microtuner for Pocketable Liquid Crystal TV Receiver" in the technical report by the Institute of Television Engineers of Japan, TEB 68-3, IPD 54-3, Jan. 29, 1981. In the shortened microstrip resonator a capacitor is connected with one or both ends of the microstrip and the length of the microstrip is shorter than that corresponding to the actual resonant wavelength, but the microstrip resonator can be resonant at a frequency corresponding to the actual resonant wavelength. In an oscillator which uses such a resonator, an overall resonant frequency f.sub.o is determined by the grounded end shortened microstrip resonator 15 (having a characteristic impedance Z.sub.1 and a length l.sub.1) and a shortening capacitor (C.sub.1) 14, a capacitor (C.sub.2) 13 represents a reactance element such as a transistor or other active device. The resonant frequency f.sub.o can be expressed by ##EQU1## where C.sub.o is a light velocity, Z=.sqroot..epsilon..multidot.Z.sub.1, and .epsilon. is an effective dielectric constant which is given by a formula (6) from the following formula: ##EQU2## where .epsilon..gamma. is a specific dielectric constant, h is a thickness of the substrate and w is a width of the microstrip line. From the formula (6), if the variations of w and h by the temperature change are equal, a variation of Z by the temperature change is essentially determined by a temperature coefficient of .epsilon..gamma.. In the formula (5), it is assumed that l.sub.1 is shorter than one-fourth (1/4) of a wavelength.
From the formula (5) it is seen that the variation of the resonance frequency f.sub.o by the temperature change is governed by the variations of the capacitors C.sub.1 and C.sub.2 by the temperature change if the variation of the substrate dimension such as Z or l.sub.1 by the temperature change is sufficiently small. Accordingly, in order to reduce the variation of f.sub.IF by the temperature change, it is necessary to optimize the temperature coefficients of the capacitors corresponding to C.sub.1 and C.sub.2 used in the first local oscillator, the narrow-band band-pass filter and the second local oscillator. Since the frequency band is in 2-3 GHz band, it is practically difficult to attain such a circuit configuration.